1. Field of the Invention
The present invention is generally related to duobinary optical signal generation and more particularly to an optical transmitter and method using half rate data streams for generating full rate modulation in a duobinary optical signal.
2. Description of the Prior Art
Recently, optical duobinary techniques have attracted attention for narrowing the spectrum of a transmitted optical signal and reducing the waveform distortion that is induced by optical fiber chromatic dispersion. The spectrum of the transmitted signal is reduced by a factor of about two by mapping a binary data signal to be transmitted into a three-level duobinary signal, with redundancy within the three levels, to represent the binary data. While there are several techniques for implementing duobinary mapping onto an optical carrier, all of the techniques result in the transmission of equivalent optical signals that take on one of three possible optical electric-field amplitude values, with certain normalization, of {tilde over ({)}1, 0, 1}.
The transmitters for generating these optical signals have electronic circuits for generating signals for driving an optical modulator. One important limitation for these electronic circuits is data rate. In general, the higher the date rate, the more difficult it is to design the circuits and the more expensive they are to manufacture. A second limitation is linearity. In general, it is less difficult and less expensive, and higher data rates are possible, when the electronic circuits are not required to be linear.
The U.S. Pat. No. 5,867,534 by Price and Uhel; and papers xe2x80x9cReduced Bandwidth Optical Digital Intensity Modulation with Improved Chromatic Dispersion Tolerancexe2x80x9d published in Electronics Letters, vol. 31, no. 1, in 1995 by A. J. Price and N. Le Mercier, and xe2x80x9c210 km Repeaterless 10 Gb/s Transmission Experiment through Nondispersion-Shifted Fiber Using Partial Response Schemexe2x80x9d published in the IEEE Photonics Technology Letters in 1995 by A. J. Price, L. Pierre, R. Uhel and V. Havard report the usage of a low-pass filter to generate the three-level duobinary signal and an optical duobinary technique where a redundancy is given to optical phase. However, because the input of the low-pass filter is the full-rate non-return-to-zero (NRZ) data, full-speed electronic circuits are required.
The U.S. Pat. No. 5,543,952; and papers xe2x80x9cOptical Duobinary Transmission System with no Receiver Sensitivity Degradationxe2x80x9d published in Electronic Letters in 1995 by K. Yonenaga, S. Kuwano, S. Norimatsu and N. Shibata, and xe2x80x9cDispersion-Tolerant Optical Transmission System using Duobinary Transmitter and Binary Receiverxe2x80x9d published in the Journal of Lightwave Technology in 1997 by K. Yonenaga and S. Kuwano report the usage of a delay-and-add circuit to generate the three-level duobinary signal and an optical duobinary technique where a redundancy is given to optical phase. Again, because the input of the delay-and-add circuit is the full-rate NRZ data, full-speed electronic circuits are required.
In both the U.S. Pat. Nos. of 5,543,952 and 5,867,534, electronic modulator drivers may operate at a bandwidth less than one-half the system data rate. However, the modulation drivers are required to be linear in order to handle the three levels of the duobinary signal.
The U.S. Pat. Nos. 5,917,638 and 6,188,497 by Franck et al, and a paper by T. Franck, P. B. Hansen, T. N. Nielsen, and L. Eskildsen entitled xe2x80x9cDuobinary Transmitter with Low Intersymbol Interferencexe2x80x9d published in IEEE Photonics Technology Letters in 1998 report a duobinary transmitter having dual binary modulation signals for driving a modulator. In a simplified view, an optical modulator is used as an adder for the delay-and-add circuit used in the U.S. Pat. No. 5,543,952. However, full-rate circuits are again required as both modulation signals have the same data rate as the optical signal.
The U.S. Pat. No. of 6,337,756; and papers xe2x80x9cA Dual-Drive Ti:LiNbO3 Mach-Zehnder Modulator Used as an Optoelectronic logic gate for 10-Gb/s Simultaneous Multiplexing and Modulationxe2x80x9d published in IEEE Photonics Technology Letters in 1992 of P. B. Hansen and A. H. Gnauck, and xe2x80x9cPrechirped Duobinary Modulationxe2x80x9d published in IEEE Photonics Technology Letters in 1998 by A. Djupsjobacka report the usage of a dual-drive modulator as both a multiplexer and a modulator. Each of the dual modulator drive signals operates at one half of the optical data rate. However, no method is proposed or successfully demonstrated for preceding the data for providing the modulator drive signals or for recovering the original data from the duobinary optical signal by symbol-by-symbol detection.
There is need for a duobinary optical transmitter using electronic circuits at low data rates without a requirement to be linear where the original data is recoverable with an optical receiver by symbol-by-symbol detection.
The present invention is a method and optical transmitter using electronic circuits operating at one-half data rate where the circuits operate without a requirement of linearity for generating an optical signal having full-rate duobinary modulation and where the original data is recoverable with an optical receiver by symbol-by-symbol detection.
Briefly, a preferred embodiment of an optical transmitter of the present invention includes a precoder and a multiplex modulator. The precoder uses two exclusive-OR gates and a one symbol delay component for calculating two cumulative cross parities for two input data streams. The multiplex modulator includes a one-half symbol delay component, modulation drivers and a dual-drive optical modulator. The one-half symbol delay component delays one of the cumulative cross parity streams by one-half symbol time with respect to the other. The modulation drivers amplify the cumulative cross parities either before or after the one-half symbol delay for driving the optical modulator. The optical modulator modulates an optical signal with a modulation drive signal corresponding to difference between the one-half symbol delayed cumulative cross parity stream-stream and the other cumulative cross parity stream for providing a duobinary optical signal having an optical electric field having an intensity that may be detected symbol-by-symbol for recovering the original data in the two input data streams.
An advantage of the present invention is that half-rate precoder and modulator driver circuits are used for generating full-rate duobinary modulation on an optical signal from which the original data can be simply detected without decoding. Because the modulator drive signals are binary, another advantage is that the modulation drivers can be operated as nonlinear amplifiers.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the best mode which is illustrated in the various figures.
FIG. 1 is a block diagram showing a duobinary transmitter of the present invention using half-rate signal processing for providing a full-rate duobinary optical signal;
FIGS. 2A-B are time charts of first and second exemplary half-rate input data streams to the duobinary transmitter in FIG. 1;
FIGS. 2C-D are time charts of first and second cumulative cross parity streams in the duobinary transmitter of FIG. 1 for the input data streams of FIGS. 2A-B;
FIGS. 2E-F are time charts of first and second modulator drive signals in the duobinary transmitter of FIG. 1 for the input data streams of FIGS. 2A-B;
FIG. 2G is a time chart of a duobinary optical electric field provided by the duobinary transmitter of FIG. 1 for the input data streams of FIGS. 2A-B;
FIG. 2H is a time chart of an intensity of the duobinary optical electric field of FIG. 2G;
FIG. 3A is a transfer characteristic for the optical electric field of a dual-drive modulator of the duobinary transmitter of FIG. 1;
FIG. 3B is a transfer characteristic for the intensity of the optical electric field of a dual-drive modulator of the duobinary transmitter of FIG. 1;
FIG. 4 illustrates an experimental setup for verifying the multiplexing and modulating functions of the duobinary transmitter of FIG. 1; and
FIG. 5 illustrates measured waveforms for the experimental setup of FIG. 4.